The present invention relates to a waveform equalizer for equalizing the waveform of a signal reproduced from a magnetic recording medium.
In a signal recording/reproduction system using a magnetic recording medium, any signal reproduced by a ring-shaped head from a longitudinal magnetic recording medium (wherein a magnetic field corresponding to the recorded data is oriented along the longitudinal direction of the medium) has a Lorentz waveform, as is shown in FIG. 3A. The discrimination of this signal is usually performed by a peak detection technique using differentiation.
On the other hand, any signal reproduced from a perpendicular magnetic recording medium (wherein the magnetic field is oriented along the perpendicular direction to a surface thereof) is point-symmetrical at about the zero amplitude, as is shown in FIG. 3B. Therefore, a zero-cross detection technique can be used for data discrimination.
The signal reproduced from the perpendicular magnetic recording medium is a composite signal consisting of the signal of FIG. 3A (referred to as "longitudinal signal" hereinafter) and the signal of FIG. 3B (referred to as "perpendicular signal" hereinafter). FIGS. 3A and 3B show ideal longitudinal and perpendicular waves, respectively. FIG. 3, however, is not ideal; rather it is a signal actually obtained in practice.
The longitudinal signal and perpendicular signal have a relation of the Hilbert transform, as V. B. Minukhin pointed out in Phase Distortions of Signals in Magnetic Recording Equipment, Telecommunications Radio Engineering, Vols. 29-30, pp. 114-120, 1975.
A method using this relationship is proposed by B. J. Langland in Phase Equalization for Perpendicular Recording, IEEE Trans. on Magn., MAG-18, pp. 1247-1249, 1982. As is shown in FIG. 1, signal fr input to Hilbert transform filter 1 is Hilbert-transformed, and is supplied to combiner 4. Signal fr is also input to delay line 2. The delayed signal is amplified by amplifier 3 and supplied to combiner 4. Combiner 4 combines the Hilbert-transformed signal with the delayed signals and a longitudinal signal. However, this reference does not disclose the detailed arrangement of the Hilbert transform filter.
A transversal filter method which is designed to realize the Hilbert transform filter and which uses a tapped delay terminated by its characteristic impedance is disclosed in U.S. Pat. No. 3,408,640 (1968) to Masson. Also, a method for converting the waveform (in a digital form) to a desired waveform is disclosed in U.S. Pat. No. 4,535,417 (1985) to Peacock. However, the Masson filter needs many taps on the delay line, and the Peacock method requires complicated circuits.
"Improvement of Recording Density by Means of a Cosine Equalizer" by T. Kameyama et al. discloses a method using a delay line whose one end is opened to generate the reflection signal. The reflection signal has the same polarity as an input signal of the delay line. Therefore, this method cannot achieve the Hilbert transform.
A small, low-cost Hilbert transform filter of the structure shown in FIG. 2 is disclosed in U.S. Pat. No. 4,635,143 (Jan. 6, 1987) by Imamura and Suzuki. Suzuki is one of the applicants of the present application. The application has been assigned to the assignee of the present application. In the waveform equalizer described and claimed in the application, it is difficult to process the reproduced signals, in which the ratios of the longitudinal signal component to the perpendicular signal component is different, from a plurality of magnetic heads. Also, the waveform equalizer has a shortcoming in that it is affected by in-phase noise.